In recent years, with increase in the production of portable and cordless equipment, expectation to small, lightweight nonaqueous-electrolyte secondary cells having high energy density has increased. As active materials for nonaqueous-electrolyte secondary cells, composite oxides of lithium and a transition metal, such as LiCoO2, LiNiO2, LiMn2O4 and LiMnO2, have been known.
Among these, especially in these days, researches for composite oxides of lithium and manganese, as highly safe and inexpensive materials, have been actively conducted, and using these as positive electrode active materials, the development of nonaqueous-electrolyte secondary cells having high voltage and high energy density, in combination with anode active materials such as carbon materials that can occlude and discharge lithium.
In general, positive electrode active materials used in nonaqueous-electrolyte secondary cells consist of composite oxides wherein a transition metal, chiefly such as cobalt, nickel and manganese, is dissolved in lithium, which is a major active material. The electrode characteristics, such as capacitance, reversibility, operating voltage and safety, depend on the kind of the transition metals to be used.
For example, nonaqueous-electrolyte secondary cells using R-3m rhombohedral rocksalt layered composite oxide, wherein cobalt or nickel is dissolved, such as LiCoO2 and LiNi0.8 CO0.2O2, can achieve as relatively high capacity density as 140 to 160 mAh/g and 180 to 200 mAh/g, respectively, and exhibits good reversibility at such a high voltage range as 2.7 to 4.3V.
However, when the cell is warmed, there are problems that the cell generates heat easily due to the reaction of the positive electrode active material with the solvent of the electrolyte during charging, or the costs of the active material are high because cobalt or nickel is expensive.
Japanese Patent Application Publication No. H10-27611 proposes, for example, LiNi0.75CO0.20Mn0.05O2 for improving the characteristics of LiNi0.8CO0.2O2, and discloses a manufacturing method utilizing the ammonium complex of the positive electrode active material intermediate thereof. Although Japanese Patent Application Publication No. H10 -81521 proposes a manufacturing method using a chelating agent of a nickel-manganese binary hydroxide material for a lithium cell having a specific grain-size distribution, no positive electrode active materials that satisfy charge and discharge capacity, cycle durability and safety at the same time can be obtained from these patent applications.
Japanese Patent Application Publication No. 2002-201028 and Japanese Patent Application Publication No. 2003-59490 propose a coprecipitated nickel-cobalt-manganese hydroxide as a material of the lithium-nickel-cobalt-manganese-containing composite oxide.
However, when the coprecipitated nickel-cobalt-manganese hydroxide is allowed to react with a lithium compound to produce the target lithium-nickel-cobalt-manganese-containing composite oxide, although reaction with lithium occurs relatively rapidly if lithium hydroxide is used as the lithium compound, in the case of using lithium hydroxide, sintering proceeds excessively by one-stage firing at 800 to 1000° C., and uniform reaction with lithium is difficult. This caused the problems of inferior initial discharge efficiency, initial discharge capacity and charge-discharge cycle durability of the obtained lithium-containing composite oxide.
In order to avoid these problems, it was required to perform firing once at 500 to 700° C., and after the fired product was crashed, to further perform firing at 800 to 1000° C. There was another problem that not only lithium hydroxide is more expensive than lithium carbonate, but also process costs for intermediate crushing, multistage firing and the like are high.
On the other hand, when inexpensive lithium carbonate was used as a lithium compound, the reaction with lithium is slow, and it was difficult to manufacture lithium-nickel-cobalt-manganese-containing composite oxide having desired cell characteristics industrially.
Japanese Patent Application Publication No. 2003-86182 proposes a method wherein a nickel-manganese-cobalt composite hydroxide is fired at 400° C. for 5 hours, mixed with lithium hydroxide, and fired. However, since this synthesizing method has disadvantages that a step for firing the material hydroxide makes the process complicated, the manufacturing costs become high, and expensive lithium hydroxide material is used.
On the other hand, although a nonaqueous-electrolyte secondary cell using a spinel composite oxide consisting of LiMn2O4 produced from relatively inexpensive manganese material is relatively hard to generate heat from the cell due to the reaction of the positive electrode active material with the electrolyte solvent during its charge, there is a problem that the capacity is as low as 100 to 120 mAh/g compared with the above-described cobalt-based and nickel-based active materials, and the charge-discharge cycle durability is poor, as well as a problem of quick deterioration at a low voltage range of below 3 V.
In addition, although there are examples that cells using LiMnO2, LiMn0.95Cr0.05O2 or LiMn0.9Al0.1O2 of an orthorhombic Pmnm system or monoclinic C2/m system have high safety, and manifest a high initial capacity, there is a problem that the crystal structure easily changes concurrent with charge-discharge cycles, and cycle durability is insufficient.
Therefore, the present invention has been devised to solve such problems, and the object thereof is to provide a highly safe positive electrode material for a nonaqueous-electrolyte secondary cell that enables the use within a wide voltage range, has a high capacity, and excels in charge-discharge cycle durability.